The present invention relates to a method for recovering fibers from white water of a papermaking process, the white water being separated from pulp mixture used for papermaking at different stages of the process, in which method the white water containing fiber material is, together with separate auxiliary pulp, conveyed to a filter such that the auxiliary pulp forms a layer on the surface of the filter and the white water flows through the layer while the fiber material remains in the layer formed by the auxiliary pulp, and in which method a mixture of the fiber material and the auxiliary pulp is removed from the filtering surface and the fiber material separated from the white water is returned to the process.
The present invention further relates to an apparatus for recovering fibers from white water of a papermaking process, the apparatus comprising a filter equipped with a filtering surface, means for feeding the white water to the filter, means for feeding separate auxiliary pulp to the filter in order to form a pulp layer on the filtering surface, means for removing the auxiliary pulp formed on the filtering surface and fiber material separated from the white water from the filtering surface, means for conveying the filtered water from the filter, and a screen for fractionating the auxiliary pulp removed from the filtering surface and the fiber material separated from the white water into two fractions having different roughness.
In the manufacturing process of a paper web or a similar fiber web, water is removed from pulp mixture at different stages so as to achieve appropriate pulp and, eventually, a dry web. The water is at least partly recycled to the process, but it is possible that some of it must be removed to a waste water unit. Such filtrate water is separated through different slotted or perforated screens and wires at different stages of the papermaking process, and the filtrate water also takes useful fiber material therewith, the fiber material typically being fine compared to the typical fiber structure of the pulp mixture.
Fiber material is ordinarily recovered from such filtrate water by different filters wherein the filtrate water is conveyed through different filtering surfaces, such as cloth or the like, such that the fiber material remains on the surface of the cloth on the input side of the water and the water is removed from the other side of the cloth. Such devices include, for example, disc filters with large discs formed from filtering elements arranged next to each other. These filtering elements comprise a frame attached on the disc filter on the axle, the element being covered by a filter bag through which the water flows into the axle through an opening located on the inside of the element at the end facing the axle. Drum filters, in which a wire operating as the filtering surface is provided on a cylindrical frame, are also used for the purpose. Such drum and disc filters and the structure and operation thereof are generally known per se to one skilled in the art; therefore, they will not be described in further detail here.
A problem with these solutions is that they should allow a large amount of water to pass through while adsorbing, however, fine fibers on the filtering surface. A solution which has been found efficient in practice is a method wherein a thin layer of pulp having longer fibers, so-called auxiliary pulp, is first built up on the filtering surface to form a thicker filtering layer on the filtering surface on which the fine fiber then remains as the water passes through the layer formed by the auxiliary pulp. The problem with the use of the auxiliary pulp in the known solutions is the feeding of the auxiliary pulp to the device since in the beginning some of the fine fiber material passes through the filtering surface before the auxiliary pulp layer has been formed. Furthermore, the structure and composition of the auxiliary pulp greatly affect its functionality and the operation of the filter. Consequently, the fiber length distribution of the auxiliary pulp greatly affects its operation as the filtering layer.
Finnish Patent FI 88732 discloses a solution wherein the auxiliary pulp to be fed to a filter is fractionated into a short fiber material part and a long fiber material part, of which only the part with the long fibers is fed as the auxiliary pulp into the filtrate water. This solution is quite difficult; furthermore, the fiber structure of the auxiliary pulp has to be such that the auxiliary pulp, as a mixture of the recovered pulp and the fine fibers, can be further fed to be used in the papermaking process.
An object of the present invention is to provide a method and an apparatus to avoid difficulties of the known solutions, and extremely efficient and functional filtering before fibers are recovered.
A method of the invention is characterized in that the mixture of the auxiliary pulp removed from the filter and the fiber separated from the white water is fractionated into a short-fibered and a long-fibered fractions, the short-fibered pulp fraction being returned to the papermaking process and the long-fibered pulp fraction being returned as the auxiliary pulp to be fed to the filter.
An apparatus of the invention is characterized in that the screen is arranged to fractionate the mixture removed from the filter such that a maximum fiber length of the pulp fraction having the shorter fibers is not greater than the fiber length of the pulp mixture used in the papermaking process, whereby the short-fibered pulp fraction can be returned to the papermaking process, and in that the apparatus comprises means for returning the long-fibered pulp fraction to the filter in order to form a pulp layer on the filtering surface.
The idea underlying the invention is that auxiliary pulp specially manufactured for the purpose is fed to a filter while the length distribution of the auxiliary pulp can be made to perfectly suit the filtering purpose regardless of what kind of pulp is used for the actual papermaking process. A further idea underlying the invention is that the auxiliary pulp is recycled in the filter such that the mixture of the auxiliary pulp and the fine fraction separated from the filter is fractionated into two parts, of which parts the one with the longer fibers is returned to be fed as the auxiliary pulp to the filter and the fraction with the shorter fibers is conveyed back to the papermaking process to be used therein.
An advantage of the invention is that it allows a fiber length to be used which is advantageous to the filtering, in which case the fiber length can greatly differ from the one used in the actual papermaking process. Furthermore, addition of a relatively small amount of the auxiliary pulp suffices since the auxiliary pulp is recycled to the filter after fractionation carried out at the outlet end, and only the part of the fibers that is to be fed into the papermaking process is then forwarded. Furthermore, the use of the auxiliary pulp does not affect the actual main process since the auxiliary pulp can, if necessary, be manufactured completely separately, which means that the main process is allowed to operate continuously without interference. A further advantage is that when the fibers used as the auxiliary pulp have been worked enough, they are transferred to the papermaking process along with the short-fibered pulp fraction separated in the fractionation, without causing any disturbance to the process. A still further advantage of the invention is that the amount of auxiliary pulp that must to be fed to the filter need only be similar to the amount that is removed with the fines during fractionation, whereby on the whole, the necessary amount of the auxiliary pulp is quite small.